Throughout the following specification, reference to a “column set” shall be understood to refer to two or more columns which act to form a section of radiation coverage over a portion of the 360° coverage area covered by the omnidirectional antenna. For example, if an omnidirectional antenna comprises three columns sets, then the antenna columns in each of the three column sets will act to cover approximately 120° of the 360° coverage area. On the other hand, if there are six column sets, then the antenna columns in each of the six columns sets will act to cover substantially 60° of the 360° coverage area of the omnidirectional antenna.
Throughout the following specification, reference to an “antenna column” shall be understood to refer to an outwardly facing component of the antenna which will mount one or more antenna radiator elements which directs the beam of radiation from the radiators.
Throughout the following specification, reference to a “radiator”, an “antenna radiator”, a “radiator element”, a “radiation element”, and/or an “antenna radiation element” shall be understood to refer to the component of the antenna which transmits/radiates the antenna beam.
At present, 2×2 MIMO omnidirectional antennas are used to transmit approximately double the amount of data over a radio frequency channel compared to a single, typical antenna arrangement. The 2×2 MIMO omnidirectional antenna arrangement achieves this doubling of throughput by using two antennas, co-located on the transmitter side, and, two antennas co-located on the receiver side. 2×2 MIMO omnidirectional antennas are deployed in the real world at present and have achieved great commercial success.
For the 2×2 MIMO omnidirectional antenna, a three- or four-sided design may be used. The three-sided type of design is shown in FIG. 1A and FIG. 1B shows the type of radiation pattern which this three-sided 2×2 MIMO omnidirectional antenna produces. The three-sided 2×2 MIMO omnidirectional antenna comprises three antenna columns 102A, 102B, 102C which each have a plurality of radiators mounted thereto and are housed within a radome 106. The 2×2 MIMO omnidirectional antenna is popular for microcell deployments, where a low power base station is used to form the microcell in a mobile phone network. The coverage afforded by the low power base station in the microcell is determined by using power control so as to limit the range of the microcell's coverage area. Depending on the frequency range being used, the typical range of a microcell is a few hundred metres and is usually less than two kilometres wide, whereas standard base stations deployed on a macrocell may have ranges of up to 40 kilometres. Referring to FIG. 1b and the radiation plot 108, it can be seen that the level of ripple, which is defined by the range of signal loss in dB between the strongest signal 110 and the weakest signal 112, is relatively small (approx. 1.5 dB) and is considered to be more than acceptable.
The 2×2 MIMO omnidirectional antenna typically consist of +/−45° polarisations or H&V polarisations. The +/−45° omnidirectional antennas are often referred to as a Pseudo Omni, or Quasi Omni, as they do not have a perfect omnidirectional pattern, which would be substantially circular in nature when viewed on a radiation polar plot. As can be seen in FIG. 1b, ripple is present on a 2×2 MIMO omnidirectional antenna pattern and this ripple causes deviation from a perfectly circular pattern. The amount of ripple can vary depending on which antenna manufacturer constructed the antenna and the construction techniques they used. In general, a +/−1.5 dB ripple would be considered to be very good and this level of ripple is shown in FIG. 1b; +/−3.0 dB ripple would be deemed to be acceptable and higher levels of ripple are not acceptable as issues will arise with coverage throughout the microcell.
As mentioned above, 2×2 MIMO omnidirectional antennas with very good or acceptable levels of ripple are commercially deployed and popular for microcells as the antenna design allows for a relatively compact antenna to fit within a radome, which is a tubular cover for the antenna, having a relatively small diameter.
Focus has now turned to 4×4 MIMO omnidirectional antennas in order to achieve a further approximate doubling of throughput again.
The development and popularity of microcells, particularly in built up urban areas, requires relatively small antennas which will not be an eyesore when installed on a side of a building or on a street lamp or power line post. Thus, it is desirable to use an antenna design which is ultra-compact yet delivers good and relatively uniform coverage across the cell by having low levels of ripple.
A commercially deployed solution for providing a 4×4 MIMO omnidirectional antenna has been to provide two 2×2 MIMO omnidirectional antennas in a physically separated arrangement. This arrangement is shown in FIG. 2A. The 4×4 MIMO omnidirectional antenna 200 of the prior art comprises two 2×2 MIMO omnidirectional antennas 100 as are known in the prior art and which are physically separated by a predefined distance 202. This predefined distance 202 is usually 10 times the wavelength (λ) of the transmission wave. This arrangement is easy to deploy but is undesirable as the overall size of the arrangement is relatively large and is widely considered to be an eyesore, particularly in urban environments.
An alternative is to use two 2×2 MIMO omnidirectional antennas which are stacked. This arrangement is shown in FIG. 2B. The 4×4 MIMO omnidirectional antenna 204 of the prior art comprises two 2×2 MIMO omnidirectional antennas 100A, 100B as are known in the prior art and which are stacked within the radome 205. This retains a relatively small radome 205 diameter, however the height of the radome 205 is doubled. Aside from the increase in height of the radome 205 which is undesirable, there are also issues with a loss of signal strength as the signal for the upper 2×2 MIMO omnidirectional antenna 100B needs to be delivered approximately one metre higher than the signal for the lower 2×2 MIMO omnidirectional antenna 100A. This extra cabling length results in approximately 0.5 dB loss in signal strength. Yet a further issue with the ‘stacked’ design approach is that the upper and lower 2×2 MIMO omnidirectional antennas 100A, 100B will have slightly different radiation polar plot patterns due to manufacturing tolerances and so on. Therefore, the coverage across the cell is not entirely uniform for each of the four ports in the stacked 4×4 MIMO omnidirectional antenna arrangement.
It has been shown that the benefits of MIMO, when using vertically stacked antenna arrays, is less than that given when the antenna arrays are deployed in a side-by-side fashion. In particular, the side-by-side antenna array shows increased data throughput and the side-by-side antenna array therefore provides higher capacity than the vertically stacked antenna arrays. Instead of stacking two 2×2 MIMO omnidirectional antennas, it has therefore been proposed to provide two 2×2 MIMO omnidirectional antenna in a side-by-side arrangement. This is shown in FIG. 2C. The 4×4 MIMO omnidirectional antenna 206 of the prior art comprises six antenna columns 210A, 210B, 210C, 210D, 210E, 210F with pairs of antenna columns 210A/210B, 210C/210D, 210E/210F arranged side-by-side to form a three-sided omnidirectional antenna housed within a radome 208. Each of the pairs of antenna columns 210A/210B, 210C/210D, 210E/210F arranged side-by-side form one of three column sets. The diameter of the radome 208 for the side-by-side approach is quite large and this is unwelcome. Moreover, the side-by-side arrangement of the radiators on the antenna columns 210A-F causes a larger ripple effect of the radiation pattern which can exceed +/−5.0 dB as is seen from FIG. 2D. The radiation plot 212 in FIG. 2D shows some acceptable signal strength 214 in some directions, but effectively null areas 216 in other directions. This is beyond the acceptable levels of ripple for microcell coverage and therefore, the 4×4 MIMO omnidirectional antennas 206 using the side-by-side arrangement are not foreseen to be tolerable for many real world deployments.
A further alternative is to utilise phase shifting to effect a 4×4 MIMO omnidirectional antenna. PCT Patent Application Number PCT/AU2011/000365 (ARGUS TECHNOLOGIES (AUSTRALIA) PTY LTD.) discloses the use of phase shifting input signals through a Butler matrix to provide a 4×4 MIMO omnidirectional antenna. In one embodiment, a six column antenna, which is arranged in a hexagonal shape, is disclosed. This hexagonally arranged set of columns each receives each of the four input signals, which have been phase shifted prior to radiation by a plurality of dual polarised antenna elements on each column. It is well known in the art that the use of such phase shifting techniques causes excessive ripple of a radiation plot and this will affect the omnidirectional nature of the antenna coverage. In the case of the hexagonally arranged six columns, each column receives each of the four input signals after the input signals have been passed through a pair of six-way Butler matrices. Such a technique will cause ripple of up to 20 dB. This can be seen from the radiation plot indicated generally by reference numeral 600, shown in FIG. 6.
It is a goal of the present invention to provide a method and/or apparatus that overcomes at least one of the above mentioned problems by providing a MIMO omnidirectional antenna which displays low, acceptable levels of ripple whilst maintaining a compact structure.